I would like all leads in my crm created from a form to also have a certain field checcked. how?

I would like all leads in my crm created from a form to also have a certain field checcked. how?

So I have a Zoho form that I use for perspective customers to fill at trade shows. This then creates a lead in my Zoho CRM.

I would like it if there is a way so that when someone fills out the form it automatically marks the field lead source as Trade show, and the name of the trade show in another field. These are both fields already created in my CRM, just not sure how to attach them to the form. 

Any and all help would greatly be appreciated. Thank you.